arXiv subjects

Banasree Sadhukhan

Publications and source records attributed to Banasree Sadhukhan.

27 records · Page 2Linked to original sources

arXiv2021

Bulk photovoltaic effect in BaTiO$_3$-based ferroelectric oxides: An experimental and theoretical study

The bulk photovoltaic effect exhibited by the non-centrosymmetric system gains research interest due to the observed large open-circuit voltage. The ferroelectric systems exhibiting anomalous photovoltaic effect are mostly crystallized with multiphase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba$_{1- x}$(Bi$_{0.5}$K$_{0.5}$)$_x$TiO$_3$ (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibits large PV response. Both experimental and theoretical studies on BBKT samples reveal ~18% reduction in bandgap compared to the parent BaTiO$_3$. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, x = 0.125 sample displays ~8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along z-direction as evidenced from the additional charge-center shift of valence band occupied by O-2p orbital and conduction band occupied by Bi-6p orbital. Notably, the degenerate Bi-p$_z$ state at conduction band minimum in BBKT favours the large shift current response in the z-direction.

cond-mat.mtrl-sci↗

arXiv2020

First principles calculation of shift current in chalcopyrite semiconductor ZnSnP$_2$

The bulk photovoltaic effect generates intrinsic photocurrents in materials without inversion symmetry. Shift current is one of the bulk photovoltaic phenomena related to the Berry phase of the constituting electronic bands: photo-excited carriers coherently shift in real space due to the difference in the Berry connection between the valence and conduction bands. Ferroelectric semiconductors and Weyl semimetals are known to exhibit such nonlinear optical phenomena. Here we consider chalcopyrite semiconductor ZnSnP$_2$ which lacks inversion symmetry and calculate the shift current conductivity. We find that the magnitude of the shift current is comparable to the recently measured values on other ferroelectric semiconductors and an order of magnitude larger than bismuth ferrite. The peak response for both optical and shift current conductivity, which mainly comes from P-3$p$ and Sn-5$p$ orbitals, is several eV above the bandgap.

cond-mat.mtrl-sci↗

arXiv2020

Tunable chirality of noncentrosymmetric magnetic Weyl semimetals

Even if Weyl semimetals are characterized by quasiparticles with well-defined chirality, exploiting this experimentally is severely hampered by Weyl lattice-fermions coming in pairs with opposite chirality, typically causing the net chirality picked up by experimental probes to vanish. Here we show this issue can be circumvented in a controlled manner when both time-reversal- and inversion- symmetry are broken. To this end, we investigate chirality-disbalance in the carbide family RMC$_2$ (R a rare-earth and M a transition metal), showing several members to be Weyl semimetals. Using the noncentrosymmetric ferromagnet NdRhC$_2$ as an illustrating example, we show that an odd number of Weyl nodes can be stabilized at its Fermi surface by properly tilting its magnetization. The tilt direction determines the sign of the resulting net chirality, opening up a simple route to control it.

cond-mat.mes-hall↗

arXiv2019

Increased metallicity of Carbon nanotubes because of incorporation of extended Stone-Wales' defects: an ab-initio real space approach

We propose an ab-initio combination of the Linear Muffin-Tin Orbital and the Recursion Methods to study the effect of extended Stone-Wales defects in single layer Carbon nanotubes. We have successfully applied this to zigzag and armchair tubes. The methodology involves no intrinsic mean-field like assumptions or external parameter fitting. As defects proliferate, the low density of states near the Fermi levels of the pristine tubes is filled with defect states. The increase of DOS at the Fermi level leads to enhanced conduction, which indicates enhanced metallicity due to SW defects in the nanotubes.

cond-mat.mes-hall↗

arXiv2017

Band-gap tuning and optical response of two-dimensional Si$_x$C$_{1-x}$: A first-principles real space study of disordered 2D materials

We present a real-space formulation for calculating the electronic structure and optical conductivity of such random alloys based on the Kubo-Greenwood formalism interfaced with the augmented space recursion (ASR) [A. Mookerjee, J. Phys. C: Solid State Phys. {\bf 6}, 1340 (1973)] formulated with the Tight-binding Linear Muffin-tin Orbitals (TB-LMTO) basis with van Leeuwen-Baerends corrected exchange (vLB) [Singh et al, Phys. Rev B {\bf 93}, 085204, (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of 2D honeycomb siliphene Si$_{x}$C$_{1-x}$ beyond the usual Dirac-cone approximation. We predicted the quantitative effect of disorder on both the electronic-structure and optical response over a wide energy range, and the results discussed in the light of the available experimental and other theoretical data. Our proposed formalism may open up a facile way for planned band gap engineering in opto-electronic applications.

cond-mat.mtrl-sci↗

arXiv2017

Effect of disorder on the optical response of NiPt and Ni$_3$Pt alloys

In this communication we present a detailed study of the effect of chemical disorder on the optical response of Ni$_{1-x}$Pt$_x$ (0.1$\leq$ x $\leq$0.75) and Ni$_{3(1-x)/3}$Pt$_x$ (0.1$\leq$ x $\leq$0.3). We shall propose a formalism which will combine a Kubo-Greenwood approach with a DFT based tight-binding linear muffin-tin orbitals (TB-LMTO) basis and augmented space recursion (ASR) technique to explicitly incorporate the effect of disorder. We show that chemical disorder has a large impact on optical response of Ni-Pt systems. In ordered Ni-Pt alloys, the optical conductivity peaks are sharp. But as we switch on chemical disorder, the UV peak becomes broadened and its position as a function of composition and disorder carries the signature of a phase transition from NiPt to Ni$_3$Pt with decreasing Pt concentration. Quantitatively this agrees well with Massalski's Ni-Pt phase diagram \cite{massal}. Both ordered NiPt and Ni$_3$Pt have an optical conductivity transition at 4.12 eV. But disordered NiPt has an optical conductivity transition at 3.93 eV. If we decrease the Pt content, it results a chemical phase transition from NiPt to Ni$_3$Pt and shifts the peak position by 1.67 eV to the ultraviolet range at 5.6 eV. There is a significant broadening of UV peak with increasing Pt content due to enhancement of 3d(Ni)-5d(Pt) bonding. Chemical disorder enhances the optical response of NiPt alloys nearly one order of magnitude. Our study also shows the fragile magnetic effect on optical response of disordered Ni$_{1-x}$Pt$_x$ (0.4$<$ x $<$0.6) binary alloys. Our theoretical predictions agree more than reasonably well with both earlier experimental as well as theoretical investigations.

cond-mat.mtrl-sci↗

arXiv2017

Effect of doping on the electronic properties of Graphene and T-graphene : A theoretical approach

In this communication we present together four distinct techniques for the study of electronic structure of solids : the tight-binding linear muffin-tin orbitals (TB-LMTO), the real space and augmented space recursions and the modified exchange-correlation. Using this we investigate the effect of random vacancies on the electronic properties of the carbon hexagonal allotrope, graphene, and the non-hexagonal allotrope, planar T graphene. We have inserted random vacancies at different concentrations, to simulate disorder in pristine graphene and planar T graphene sheets. The resulting disorder, both on-site (diagonal disorder) as well as in the hopping integrals (off-diagonal disorder), introduces sharp peaks in the vicinity of the Dirac point built up from localized states for both hexagonal and non-hexagonal structures. These peaks become resonances with increasing vacancy concentration. We find that in presence of vacancies, graphene-like linear dispersion appears in planar T graphene and the cross points form a loop in the first Brillouin zone similar to buckled T graphene that originates from $π$ and $π$* bands without regular hexagonal symmetry. We also calculate the single-particle relaxation time, $τ(\vec{q})$ of $\vec{q}$ labeled quantum electronic states which originates from scattering due to presence of vacancies, causing quantum level broadening.

cond-mat.mes-hall↗

arXiv2017

Disorder induced lifetime effects in binary disordered systems : a first principles formalism and an application to doped Graphene

In this work the conducting properties of graphene lattice with a particular concentration of defect (5\% and 10\%) has been studied. The real space block recursion method introduced by Haydock et al. has been used in presence of the random distribution of defects in graphene. This Green function based method is found more powerful than the usual reciprocal based methods which need artificial periodicity. Different resonant states appear because of the presence of topological and local defects are studied within the framework of Green function.

cond-mat.mtrl-sci↗

arXiv2015

Configuration and Self-averaging in disordered systems

The main aim of this work is to present two different methodologies for configuration averaging in disordered systems. The Recursion method is suitable for the calculation of spatial or self-averaging, while the Augmented space formalism averages over different possible configurations of the system. We have applied these techniques to a simple example and compared their results. Based on these, we have reexamined the concept of spatial ergodicity in disordered systems. The specific aspect, we have focused on, is the question "Why does an experimentalist often obtain the averaged result on a single sample ?" We have found that in our example of disordered graphene, the two lead to the same result within the error limits of the two methods.

cond-mat.mtrl-sci↗